The present invention relates to a semiconductor device such as a free wheel diode for use in reverse parallel connection with an insulated gate bipolar transistor (IGBT).
In recent years, IGBTs have significantly advanced to achieve a low on-voltage characteristic and a fast-switching characteristic (a reverse recovery characteristic) and are approaching their theoretical limits. Under the circumstances, much attention is being paid to the characteristics of diodes integrated into IGBT modules. Such diodes require a low ON-voltage and fast-switching capabilities, but the focus is placed on the dependency of the ON-voltage on temperature. This is because the recent increase in the size of the IGBT module has increased the number of diode chips integrated into the module and operating in parallel.
If a semiconductor converter with an IGBT module mounted thereon fails for any reason, a large current flows through the IGBT module and naturally through the diodes therein. If the diodes have a negative temperature coefficient, that is, are characterized in that their ON-voltage decreases with increasing temperature, then of the plurality of diodes connected in parallel, those through which a large current flows undergo an increase in temperature, reducing the ON-voltage to cause a much larger current to flow through. Finally, those diodes in which the current concentrates are destroyed.
The diodes include pn diodes formed using simple pn junctions and Shottky diodes that are unipolar elements. The pn diodes involve injection of minority carriers, so the diffusion potential of the pn junction decreases with increasing temperature. In addition, in order to improve the switching characteristic, a lifetime killer is normally introduced into the semiconductor using heavy metal or electron beams to shorten lifetime. The lifetime killer, however, has its effects weakened as the temperature increases. These phenomena serve to reduce the ON-voltage as the temperature increases. In addition, it is difficult to make this decrease in ON-voltage uniform among the diodes, and the temperature characteristic varies among the diodes. This variation may subject some diodes to current concentration, as described above.
On the other hand, due to the lack of injection of minority carriers, the Shottky diodes undergo an increase in ON-voltage as the temperature increases, and have a high switching speed. These diodes, however, have a high ON-voltage value.
A. Prost et al. have reported in Proc. of IEEE ISPSD ""97 pp. 213-216 (1997) that the switching characteristic can be improved by reducing the concentration of impurities in the anode layer to reduce the diffusion depth of this layer and introducing a lifetime killer to make the temperature coefficient of the ON-voltage positive. In addition, M. Mori et al. have reported in Proc. of IEEE ISPSD ""91 pp. 113-117 (1991) that the switching characteristic can be improved by connecting Shottky and pn diodes together in parallel inside a single cell and restraining injection of minority carriers from the anode layer to make the temperature coefficient of the ON-voltage positive.
In the above pn diodes, however, since the concentration of impurities in the anode layer has been reduced to reduce the diffusion depth, application of a reverse bias voltage to the pn diode causes the anode layer to be punched-through with a low voltage, thereby hindering a withstand voltage from being obtained.
In addition, in the combination of Shottky and pn diodes, due to the very small diffusion depth of the Shottky diode portion and the use of a (p) layer having a low concentration of impurities, the Shottky barrier height cannot be controlled easily, thereby varying the ON-voltage or its temperature or switching characteristic.
In view of the above, it is an object of the present invention to solve these problems and to provide a semiconductor device that has a positive ON-voltage temperature coefficient and a high switching speed at the current density provided during actual operations.
In order to attain the above object, the present invention provides a semiconductor device comprising a first conductivity-type base layer having high resistance, a second conductivity-type anode layer formed on one surface of the first conductivity-type base layer, an anode electrode formed on a surface of the second conductivity-type anode layer, a cathode layer formed on the other surface of the first conductivity-type base layer, and a cathode electrode formed on a surface of the cathode layer, wherein said anode electrode is secured to a part of the second conductivity-type anode layer, and the ratio of areas S1/S2 is between 5 and 30, where S1 is the area over which said anode electrode is not secured to the second conductivity-type anode layer, and S2 is the area in which the anode electrode is not secured to the second conductivity-type anode layer.
An insulating film is interposed between the second conductivity-type anode layer and the anode electrode so that the second conductivity-type anode layer and the anode electrode are not fixed together.
The semiconductor device has a plurality of spaced areas over which the second conductivity-type anode layer and the anode electrode are fixed together, and these areas constitute stripes, arcs, rings, or islands.
The second conductivity-type anode layer, the first conductivity-type base layer, and the first conductivity-type cathode layer are preferably subjected to electron beam irradiation or heavy metal diffusion.
By contacting the main electrode only with a portion of the second conductivity-type anode layer as described above, a current flowing through the device flows across the second conductivity-type anode layer. Due to the high concentration of impurities, the second conductivity-type anode layer performs almost unipolar operations while the device is ON. Thus, with an increase in temperature, the mobility and diffusion coefficient of the second conductivity-type anode layer decrease, thus causing the resistance (of MOSFETs or other devices) to increase. Consequently, the ON-voltage has a positive temperature coefficient.
In addition, the switching speed can be increased by introducing a lifetime killer by means of electron beam irradiation or heavy metal diffusion.